In mineral, e.g. coal, mining installations in underground mine workings, it is well known to provide each roof support with a control unit forming part of an electro-hydraulic control system for the installation. This control unit, which normally has a microprocessor, serves to energize a large number of electromagnetic valves which function to control the flow of hydraulic pressure fluid to various devices. In the harsh environment in the working, the valves are prone to fail from time to time and this can have serious consequences. There is a need to detect the failure of a valve as soon as possible so that it can be replaced. In one known monitoring apparatus described in German patent specification No.2923694 luminous diodes are provided in the energizing circuits of the coils of the valves. This provides a reliable indication of whether the coil of a valve is open circuit but this is only one fault condition which can occur. This simple technique does not provide any indication as to whether the spools or armatures of the valves are being displaced correctly or whether a valve has become jammed in one switching state. German patent Specification No. 2602906 describes monitoring apparatus in which a signal is generated when the armature of a valve reaches its switching positions. If the signal occurs outside a certain allowable time period then a fault condition prevails. This monitoring technique thus relies essentially on the response time of the armature and detects when the current through the coil drops.
A general object of the present invention is to provide an improved monitoring method and apparatus.